Because of the trend towards miniaturization of electronic devices, higher power dissipation in small packages and thus higher heat fluxes have become a more critical problem. In order to achieve long life and high reliability for electronic systems, it is absolutely essential that adequate cooling be provided for electronic packages.
Systems for cooling electronic components, such as semiconductor chips are well known and take a variety of forms. For example, U.S. Pat. No. 4,712,158, relates to a cooling system for electronic components mounted on a circuit board in which liquid coolant can be completely discharged from a cooling plate when it is necessary to have access to the electronic components. Hollow resilient members are mounted at the open end of a coolant chamber and comprise bellows and a heat transfer piece mounted at the ends of the bellows so as to be in direct and resilient contact with the electronic components. A nozzle pipe protrudes into the coolant chamber toward the heat transfer piece for directing the liquid coolant thereagainst. Coolant can be removed from the cooling plate so that no coolant remains in the chamber, thereby allowing the components to be removed for maintenance purposes or for transfer. However, such a system does not maximize fluid movement within the bellows to minimize stagnation and to enhance heat transfer. Furthermore, the disclosed embodiments direct the cooling fluid against a single heat transfer piece and thus do not maximize the heat transfer area so as to allow dissipation of large quantities of heat which is particularly important for power components.
U.S. Pat. No. 4,729,076, discloses several embodiments of a cooling system using an expandable bellows for an electronic circuit device. One such arrangement uses a cooling module which has a heat transfer plate connected to a header within which a cooling passage permits a coolant to flow by means of a bellows. The heat transfer plate is exposed to the flow of the coolant and is pressed against an electronic circuit component such as a semiconductor provided on a circuit board by means of the bellows actuated by a hydraulic coolant pressure. Another embodiment disclosed in that patent shows a heat transfer plate connected to a header by means of a bellows. A heat transferring compliant member is provided between the heat transfer plate and a circuit component by securing the compliant member to the outer side face of the heat transfer plate or to the circuit component on the printed circuit board. The compliant member is preferably made of an elastic material having a high heat conductivity. A liquid or gas coolant which is circulated by a pump enters a coolant recirculation zone defined in the bellows so that the heat transfer plate is exposed to the coolant at the side face thereof opposite to the compliant member. Although such an arrangement accommodates surface irregularities or misalignment between the cooling device and the electronic circuit, the several embodiments do not provide the advantages which impingement jet cooling provides in terms of movement of fluid to increase the heat transfer coefficient and eliminate stagnation at boundary layers. Furthermore, it does not enhance the area through which heat transfer occurs and thus the efficiency of the system. This is true even though the cited patent also shows the use of a heat transfer stud inside the bellows with cooling fins at the upper end thereof. In addition, there is only a single bellows provided per cooling device. Should there be any leakage in or around the bellows, the effectiveness of the cooling device is entirely lost for that electric component and, depending upon the seriousness of the leakage, possibly the other cooling devices for other components.
U.S. Pat. No. 4,740,866 discloses a sealed type liquid cooling device with an expandable bellows for semiconductor chips. Specific attention is paid to the assembling process shown in FIG. 3 in which a Kovar thin film 1 is coated with a photoresist film and is used as the bellows material. Pressure is applied to join the outer peripheral junction rings and the bellow rings after which the bellow parts are cut and removed and the bellows are stretched. The assembled bellows adjoined to a housing and a cap for a cooling block. The cooling block and the silicon chip are metallically joined by solder. It does not appear that the bellows is intended to expand to and from the silicon chip to be cooled. Whereas a pipe can be used to introduce cooling water into the bellows, this arrangement does not use any wicking devices within the bellows to more rapidly perform heat transfer.
U.S. Pat. No. 4,138,692, shows several embodiments of bellows cooling in which a thermally conductive solid stud is in thermal contact with the bellows. It has been found that such a thermally conductive stud does not provide a satisfactory cooling efficiency when dealing with components generating a substantial amount of heat in that it does not enhance the effective heat transfer area. Furthermore, this arrangement is one in which the stud is to be permanently affixed to the chip and is not intended to be moved to and away from the chip so as to afford the opportunity to replace the chip.
U.S. Pat. No. 4,561,041, also shows a bellows-like cooling system for chips. This cooling system provides an outer and inner bellows which are rigidly connected with a heat sink which, in turn, is bonded in a rigid fashion to a heat sink cooling chip provided with narrowly spaced grooves on an upper side thereof. Glass plates having axially and radially extending ducts are cut into a plate and act as fluid passageways. Again, this arrangement suffers from the fact that it is not intended to be movable up and down and does not provide means inside the bellows to facilitate heat transfer by enhancement of the cooling area. Furthermore, although this patent shows the use of an inner and outer bellows, such an arrangement is not intended to provide a redundancy in the event of leakage of the cooling fluid. Additionally, it is not intended that the bellows be moved into and out of contact with the chip for replacement purposes.
The foregoing problems have been overcome by the provision of a laminated stack of highly conductive orifice plates separated by spacer plates and arranged with a bellows. The orifice plates serve as target plates for impingement jets caused by pressurized fluid coolant issuing from orifices of the immediate upstream plate. Alternate orifice plates have offset or staggered hole patterns so that the impingement jets are directed at the solid surface between the orifices of the immediate downstream plate.
An object of the present invention is to use the area-enhancement advantage of impingement jet cooling in areas where little space is available and in a manner by which electronic modules can be easily replaced.
In order to carry out that object, a laminated stack of plates is arranged with a metal bellows. Fluid coolant flows through the stack, and the metal bellows expand when the coolant is initially pressured to move the cooler assembly into contact with the electronic module which acts as the heat source. A thermally conductive interface elastomer material such as CHO-THERM.TM. interfaces between the bellows and the module so that any surface irregularities on the module or misalignment between the assembly and module are accommodated without impeding the transfer of heat. When the coolant circuit is depressurized, the spring constant in the bellows will cause the assembly to retract from the module allowing servicing or replacement.
Another object of the present invention is to provide a cooling device which has both a low thermal resistance to a liquid coolant and which is also a separable module.
It is a further object of the present invention to be able to cause the cooling device to move towards and away from the electronics module by activation of the coolant pressure so that the electronics package can be removed when the fluid circuit has been depressurized.
It is still a further object of the present invention to provide a cooling device which is both simple in construction and yet which is highly efficient in its heat removal capability.
It is yet another object of the present invention to provide a coolant activated device where surface irregularities or misalignment between the cooling device itself and the heat source are accommodated by an elastic contact pad or a thermally conductive elastomer such as CHO-THERM.TM., a thermal interface material offered by Chomerics, 16 Flagstone Drive, Hudson, NH 03051.
Another object of the present invention is to provide cooling by the use of impingement jets on orifice plates which serve as target plates so as to give extremely high heat transfer coefficients.
Still a further object of the present invention is to provide a coolant activated cooling device wherein the high heat transfer coefficients in combination with the area enhancement afforded by a stack of thin laminate layers provides a very low heat source-to-heat sink thermal resistance.